Good food for thought: Clancy Ratliff posts a wonderful "Collection of Good and Not-So-Good Reasons for Assigning a Personal Narrative as the First Essay in a Composition Course" on the CultureCat weblog.
If I understand it correctly, Ratliff is responding to a lecture by Bruce Horner that suggested that the motivations for assigning personal essays often contradict or muddle up the rhetorical task. I haven't heard Horner's argument, but I'd suggest that the multiplicity of rationales is actually a sign that the assignment is a rich one, operating on multiple levels and therefore meeting multiple student needs.
I assign personal narratives often at the beginning of a term. I see my motives in virtually all of the reasons Ratliff posts...the only motive not mentioned that I can think of is that it serves a "de-icing" function by humanizing the institution, inviting students to self-express to thaw out the chill of fear early in the term. It just seems like the most honest way to begin. It also can encourage a habit of critical journal writing, if that's a method used in the course. One of the difficulties I have is not assigning or assessing these papers; its weaning some students from writing too informally later in the term, when formal research papers are due. The struggle with academic voice victimizes the style and makes a mess out of things. But it's a good struggle, I think, ultimately.
Really appreciate this point, Mike:
"...it serves a "de-icing" function by humanizing the institution..."
Theory and professional tactics aside, every success I've ever had with real students has come on the heels of humanizing the experience (which becomes the foundation for more rigorous/formal work in time).